This invention pertains to a failure override system for an hydraulic pump or motor displacement control whereby, upon a failure in the system, the pump or motor is set in a nondisplacement or displacement position, respectively, which minimizes the adverse effects resulting from the failure.
Hydrostatic transmissions are now well known wherein a pump and motor are connected in fluid circuit and with one or both of the pump and motor having variable displacement whereby the characteristics of the system, such as the output speed of the motor, may be varied. Such hydrostatic transmissions have many different uses, with one use thereof being in the drive train for a vehicle. An example of use of two such transmissions for driving a dual track vehicle is shown in MacIntosh et al U.S. Pat. No. 3,946,560, owned by the assignee of this application.
There are many different known methods for setting the displacement of a variable displacement pump or motor. Common to many of these systems is the use of a control valve which controls the connections of a pair of positioning piston and cylinders associated with the swash plate to either a source of control pressure or drain. This control valve can be positioned mechanically, hydraulically, or electrically and may or may not have feedback means associated therewith for feeding back the position of the swash plate to the control valve. In many instances, it is possible for a failure to occur in a component associated with positioning of the control valve, with the result that the control valve can move to a position to cause swash plate positioning which would have adverse effects. If a failure occurs in the control for the pump, it is possible for the control valve to move to a position whereby the swash plate would move rapidly to and remain at a full displacement position which could cause a rapid rise in the speed of the motor and create hazardous conditions as by causing a vehicle utilizing the transmission to inadvertently increase in speed.
In the control of a motor, particularly when the hydrostatic transmission is associated with a vehicle for drive thereof, it is desirable to have the motor go to a maximum displacement position upon a failure in the system.
Examples of possible failure are failure of a spring in a system having springs utilized to provide for spring centering of the control valve, a failure in the hydraulic or electrical system for positioning of the control valve, or a failure caused in a mechanical system, such as breaking of a link or a pivot point in the structure which sets the position of the control valve or in the feedback structure which relates the position of the control valve to that of the swash plate and wherein spring means are associated with the feedback structure to urge the structure in a direction to eliminate undesirable effects of backlash.